The aim of this work is to characterize the effects of randomly induced mutations on fitness in populations of Drosophila melanogaster. Mutations will be induced on the X-chromosome with ethyl methanesulfonate (EMS) and x-rays. Their effects will be ascertained in single and multiple generation experiments. In the former, the influences of the mutants on survival will be measured; in the latter, attention will be focused on total fitness effects. Of course, total fitness includes survival, but it involves other components, such as fertility, as well. One goal is to determine the connection between the survival and total fitness effects of new mutants. Another is to compare the effects of chemical- and radiation-induced mutations. A third is to explore the possibility that the total fitness effects vary from one generation to another, and if so, randomly or otherwise. In a separate series of experiments using EMS-induced and spontaneous mutants, data will be collected to determine if fitnesses are transitive; these experiments will be of the multiple generation type. In much of this work the method will involve competition between mutagenized and unmutagenized C chromosomes carried in hemizygous males. By monitoring the relative frequencies of the two over time, it is possible to estimate the intensity with which mutants are selected out of the population. Taken together with known mutation rates, these estimates should enhance our understanding of how mutation and selection affect the level of genetic variability in Drosophila populations.